Add a Twitter service

[ericflo]

While Facebook, Instagram, and Google Photos are good, I also upload many photos to Twitter, and many of them aren't uploaded anywhere else. Mediasummon should be able to connect to and backup my Twitter timeline too.

This is slightly trickier than it seems at first, because all the current services authenticate using OAuth2, but Twitter only supports OAuth1 authentication. So, OAuth1 support will have to be built, and there may be hidden assumptions we made when implementing things that assumed OAuth2, that we'll need to change.

But change they must, because supporting Twitter would be very nice.